1. Field of the Invention
The present invention relates to a twisted nematic (TN) liquid crystal display device and, more particularly, to a TN liquid crystal display device which is improved in terms of visual angle dependence of contrast and color in halftone display.
2. Description of the Related Art
Recently, TFT-TN liquid crystal display devices have been used as displays for wordprocessors, personal computers, and the like. In such a liquid crystal display device, a polarizer is normally arranged on the incident side of a TN liquid crystal cell, in which a thin-film transistor (TFT) for a drive operation is arranged for each pixel, in such a manner that its light-transmitting axis is parallel to the aligning treatment direction of the incident-side substrate of the liquid crystal cell, whereas an analyzer is arranged on the exit side of the liquid crystal cell in such a manner that its light-transmitting axis is almost perpendicular to the light-transmitting axis of the polarizer. In this liquid crystal display device, each pixel can be driven by applying a static voltage to it. For this reason, the liquid crystal display device of this type exhibits higher contrast and a larger viewing angle than a simple matrix type liquid crystal display device.
However, the viewing angle of such a conventional TFT-TN liquid crystal display device is smaller than that of a CRT as a general-purpose display. In addition, in halftone display, changes in color occur in accordance with changes in visual angle (angle of gaze with respect to the normal line of a display surface), i.e., the visual angle dependence of changes in color is conspicuous.
FIG. 1 shows equi-contrast curves obtained when applied voltages are set as V=0 [V] and V=4.38 [V] in a typical conventional TN liquid crystal display device. Referring to FIG. 1, the concentric circles respectively represent, from the innermost circle, visual angles of 10.degree., 20.degree., 30.degree., 40.degree., and 50.degree. with respect to the normal direction of a substrate of the liquid crystal display device. In addition, each black square ( ) represents that the contrast is 10; each white square (.quadrature.), 50; each black triangle ( ), 100; and each white triangle (.DELTA.), 150. An arrow R indicates the aligning treatment direction of the incident-side substrate. According to FIG. 1, the visual angle direction in which the contrast is high is the downward position at which an angle representing the azimuth of the display surface (to be referred to as an azimuth angle hereinafter) is 315.degree. with reference to the aligning treatment direction R of the incident-side substrate, and the regions in which the contrast is high expand in the leftward and rightward directions corresponding to azimuth angles of 225.degree. and 45.degree., which are apparently brighter than the region expanding in the upward and downward directions corresponding to azimuth angles of 135.degree. and 315.degree.. In addition, the region expanding in the downward direction has higher contrast than that of the region expanding in the upward direction. Although no inversion region is present near the region expanding in the upward direction, this region corresponds to the visual angle direction in which the contrast is lowest.
FIGS. 2A to 2D show the visual angle dependence of Y-V curves representing the relationship between a transmittance Y and an applied voltage V. As shown in FIG. 2A, if the visual angle is changed within the ranged of 0.degree. to 50.degree. in the downward direction corresponding to an azimuth angle of 315.degree., a large lump portion appears in a range of the Y-V curve which corresponds to V=2.0 to 4.0 [V]. Since an applied voltage range in which halftone display is performed corresponds to V=1.5 to 4.0 [V], if halftone display is performed, a phenomenon of brightness inversion becomes conspicuous.
As shown in FIGS. 2B and 2C, the above-mentioned phenomenon does not occur in the rightward direction corresponding to an azimuth angle of 45.degree. on the display surface and in the leftward direction corresponding to an azimuth angle of 225.degree.. As shown in FIG. 2D, however, if the visual angle is changed within the range of 0.degree. to 50.degree. in the upward direction corresponding to an azimuth angle of 135.degree., a Y-V curve is gradually moderated, and the difference of brightness between halftone levels is reduced to zero.
Prior to a description of the visual angle dependence of changes in color in the conventional TN liquid crystal display device, a color difference .DELTA.E*, a brightness index difference .DELTA.L*, and a chroma difference .DELTA.C* will be described first. The color difference .DELTA.E* means "the distance between display colors", regarding "display colors" at the front visual angle as standard colors. The color difference .DELTA.E* is determined by the brightness index difference .DELTA.L* and the chroma difference .DELTA.C*. These physical amounts are defined in a CIE 1976 (L*, u*, v*) colorimetric space.
The following are conversion formulae for converting values from a normal (X, Y, Z) colorimetric space to the CIE 1976 (L*, u*, v*) colorimetric space: ##EQU1##
FIGS. 3A to 3F, 4A to 4F, and 5A to 5F are graphs respectively showing the visual angle dependence of the color difference .DELTA.E*, the brightness index difference .DELTA.L*, and the chroma difference .DELTA.C* in four directions, i.e., upward, downward, leftward, and downward directions (azimuth angles: 135.degree., 315.degree., 225.degree., 45.degree.) for six different applied voltages. In this case, the six different applied voltages are: 0 V, 1.5 V, 2 V, 2.5 V, 3 V, and 4 V. In each graph, a white square (.quadrature.), a plus (+), a white circle (.smallcircle.), and a white triangle (.DELTA.) indicate values obtained when the visual angle is changed in the upward, downward, leftward, and rightward directions, respectively.
As is apparent from these graphs, in the conventional TFT-TN liquid crystal display device, the difference between "display colors" with respect to changes in visual angle in halftone display is undesirably large. Under the circumstances, it is required for the conventional TFT-TN liquid crystal display device to improve the visual angle dependence of contrast and display color (to be referred to as visual angle characteristics hereinafter) so as to accurately display images, especially halftone images, in multi-gradation display regardless of visual angles.